Method for synchronizing the internal clock of a mobile radio terminal with local time

ABSTRACT

The invention relates to a method for synchronising the internal clock of a mobile radio terminal with the local time, which is characterised in that the mobile radio terminal transmits a short message (SM) to itself, the local time may be deduced from the transmission and received times for the short message signal and the time stamp within the received short message and the internal clock is synchronised with the said local time.

[0001] The invention relates to a method for the synchronization of theinternal clock of a mobile radio terminal device with the local time.

[0002] It is known that a mobile radio terminal device, in particular aGSM terminal device, does not have the capability of determining thecorrect local time of day. Even if a clock module is installed in theterminal device, it is, in general, necessary for the user to provide itwith the correct time.

[0003] The invention addresses the problem of proposing a simple andautomated method for the synchronization of the internal clock of amobile radio terminal device with the local time.

[0004] The problem is solved according to the invention through thecharacteristics of patent claim 1.

[0005] Advantageous embodiments and further developments of the methodaccording to the invention are specified in the dependent patent claims.

[0006] The described method offers the mobile radio subscriber thecapability of having displayed the current local time of day by themobile radio terminal device in simple manner. The time of day can bedetermined accurately to a few seconds, which, for practical purposes,is entirely sufficient.

[0007] The method can be expanded such that a mobile radio subscribercan be provided with the “correct” local time even when traveling, i.e.when roaming, virtually by the “push of a button”.

[0008] In conjunction with FIGS. 1 and 2 an example of the methodaccording to the invention will be explained. Based on the Figures andtheir description, additional characteristics and advantages of theinvention are evident. In the drawing depict:

[0009]FIG. 1 by way of example a flow chart of the method for settingthe clock in the terminal device by means of a short message to itself,

[0010]FIG. 2 flow chart for improving the accuracy of the method.

[0011] As is evident in FIG. 1, the method according to the inventionessentially utilizes the fact that in digital mobile radio networks, inparticular GSM mobile radio networks, each short message is conductedvia a so-called Short Message Service Center (SMSC), and here eachmessage is provided with a time stamp t_(Z) which, in turn, is indicatedto the receiver of the message.

[0012] Thus, if a mobile radio terminal device sends a short message toitself at time t₀, it is informed at time t₂ (after a few seconds) ofthe time t_(Z) of the arrival of its own message in the SMSC whenreceiving the short message sent to itself. If the terminal deviceremembers the sending time t₀ by registering the setting of the internalclock, as well as the acknowledge time t₁, a few seconds after sendingits own message, then thereby a time window between t₀ and t₁ is given,in which the time stamp t_(Z) of the SMSC must assuredly lie. Throughmaximal boundary definition of the time window, the time t_(Z) can bederived with good accuracy.

[0013] A detailed consideration of the minimum transmission times in theGSM network, in particular the delay time of the signals at the airinterface, permits a further shrinking of the time window between t₀ andt₁, such that an accuracy of the prediction of t_(Z) of approximatelyone second appears attainable. This accuracy is often sufficient forpractical purposes. This will be explained in the following inconjunction with FIG. 2.

[0014] At time t₀ the terminal device sends a short message to itself,which is received by the SMSC.

[0015] The terminal device calculates the time t₀′=t₀+d₀ wherein d₀ isthe estimated or known minimum transmission time for the message fromthe terminal device to the SMSC. The message cannot have arrived at theSMSC before the passage of d₀ time units.

[0016] The terminal device waits for the acknowledgment (Ack) of thereception of the message in the SMSC. The acknowledgment arrives at timet₁.

[0017] The terminal device calculates the time t₁′=t₁−d₁, wherein d₁ isthe estimated or known minimum transmission time for the acknowledgmentfrom the SMSC to the terminal device. The SMSC must have gotten theacknowledge on its way before this time t₁′.

[0018] Now, the mean value t_(M)=(t₁′+t₀′)/2 is formed from thecalculated times t₀′ and t₁′. This mean value t_(M) corresponds fairlyaccurately to the value (point in time) of the time stamp t_(Z) assignedby the SMSC.

[0019] The terminal device now waits for the arrival of the messagedirected to itself, which arrives at time t₂.

[0020] The terminal device takes the time stamp t_(Z) from the message.

[0021] The terminal device compares the time stamp t_(Z) with thecalculated value t_(M) and, if necessary, corrects its internal clock bythe difference of t_(D)=t_(Z)−t_(M) time units. It is obvious that withthis selection of correction the maximum time error relative to the SMSCis t=(t₁−t₀)/2 or the least.

[0022] The absolute accuracy depends on the maximum discrepancy of theSMSC clock from the “true time”, i.e. the absolute time error is greaterby the inaccuracy of the time base in the SMSC.

[0023] The method can also be applied in the case of roaming, i.e. if amobile radio subscriber is located outside of his home network, if theSMS message is transmitted to a time server connected to the SMSC, andthe time server is informed in the message of the network code of thatnetwork into which the terminal device is currently logged in. The timeserver provides in his response to the terminal device information aboutthe time zone or the correction to be applied. Herein the home SMSC cancontinue to be employed. This can be realized relatively readily in theterminal device, for example through an SIM toolkit application.

1. Method for the synchronization of the internal clock of a mobileradio terminal device with the local time, characterized in that themobile radio terminal device sends a short message (SM) to itself, fromthe sending and receiving times of the short message signals and a timestamp t_(Z) comprised in the received short message, derives the localtime of day and synchronizes the internal clock with this local time ofday.
 2. Method as claimed in claim 1, characterized in that the mobileradio terminal device registers the sending time t₀ of the short messageas well as the time of acknowledgment t₁ of the short message andtherefrom derives the time t_(M)=(t₁+t₀)/2, wherein t_(M) correspondsapproximately to the point in time of the time stamp t_(Z).
 3. Method asclaimed in one of claims 1 or 2, characterized in that the mobile radioterminal device extracts from the received short message the time stampt_(Z), compares it with t_(M), and corrects the internal clock by thedifference t_(D)=t_(Z)−t_(M).
 4. Method as claimed in one or several ofthe preceding claims, characterized in that an improvement of theaccuracy of t_(M) is attained by taking into consideration the delaytimes d₀ and d₁ with t_(M)=(t₁′+t₀′)/2, wherein t₁′=t₁−d₁ and t₀′=t₀+d₀.5. Method as claimed in one or several of the preceding claims,characterized in that the SMSC is informed in the short message of thenetwork code of that network into which the terminal device is currentlylogged in, which network code is transferred to a time server connectedto the SMSC and the time server transmits to the terminal device in itsresponse the time zone or the correction to be applied.